1. Field of the Invention
The present invention relates to a battery, a lead member for battery connection (hereinafter referred to as lead member) welded to the outer surface of the battery by the parallel resistance welding method, and a battery pack using the same, and more specifically, to a battery and a lead member having surface conditions such that high weld strength can be realized in a stable state between the lead member and a battery container of the battery when the lead member is welded to the outer surface of the battery, and a battery pack using the same.
2. Prior Art
With the spread of various electric and electronic devices, an increasing number of devices have started to be designed so that they directly incorporate a battery pack that includes a plurality of connected batteries as drive sources. Charging or discharging the batteries of these devices requires use of lead members for electrical connection between the batteries and between package terminals and the batteries. Thus, a lead member must be fixed to the outer surface of each battery, e.g., a part of the surface of its container, such as the cover or bottom portion thereof.
FIGS. 1 and 2 show an example of the battery pack.
FIG. 1 is a perspective view showing an outline of the battery pack. This battery pack A is composed of a battery group (mentioned later) stored in a shell 1 that is formed of a material such as a polycarbonate resin or polycarbonate/acrylonitrile-butadiene-styrene resin. A positive terminal 6A and a negative terminal 6B are exposed from the shell 1. Also exposed is a signal fetch terminal 6C for a built-in thermistor for the temperature control of the battery group, for example.
FIG. 2 is a perspective view schematically showing a battery group B. The battery group B includes three cylindrical batteries 2A, 2B and 2C that are connected in series with one another. More specifically, the bottom (negative terminal) of the battery 2A and the positive terminal of the battery 2B are connected by means of a lead member 3A, while the bottom (negative terminal) of the battery 2B and the positive terminal of the battery 2C are connected by means of a lead member 3B. One end of a lead member 3C is connected to the positive terminal of the battery 2A, and the other end to the positive terminal 6A of the battery pack A. One end of a lead member 3D is connected to the bottom (negative terminal) of the battery 2C, and the other end to the negative terminal 6B of the battery pack A.
Further, a thermistor 4 is interposed between the batteries 2A and 2B, and two signal lines 5A and 5B are led out of the thermistor 4. The one signal line 5A is connected to the signal fetch terminal 6C of the battery pack A, and the other signal line 5B to the lead member 3D.
It is to be understood that insulating members (not shown) are interposed as required between the batteries, between the batteries and the thermistor, etc. lest the batteries and the thermistor be shorted.
Secondary batteries, such as nickel-cadmium secondary batteries, nickel-metal-hydride secondary batteries, lithium-ion secondary batteries, are frequently used as the batteries that constitute the battery group B.
In general, each of these secondary batteries is constructed so that a specific power generating element is stored together with a prescribed electrolyte solution in a battery container, which is composed of an open-topped battery can and a cover member closing the top opening of the can.
Conventionally, the battery can, especially for alkaline batteries, is formed of an Ni-plated steel sheet. It is obtained by shaping a sheet of soft steel, such as very-low-carbon steel, into a specific bottomed can by plastic deformation and forming an Ni layer of a desired thickness on the surface of the resulting structure by Ni-plating for the purpose of rustproofing. The cover member is obtained in like manner.
Generally, on the other hand, each of the lead members that constitute the battery group B is formed of a small strip of Ni or, like the battery container, is formed of an Ni-plated steel sheet.
The following parallel resistance welding method is generally applied to the connection between the batteries and the lead members for the manufacture of the battery group B.
First, a lead member 3 to be welded is put on a surface 2a (bottom surface as illustrated) of a battery container 2, as shown in FIG. 3.
Two welding electrodes 7, each having a small-diameter distal end 7a, are arranged parallel to each other with a given space between them on a surface 3a of the lead member 3. A predetermined pressure loading from the welding electrodes 7 is applied to the lead member 3 so that a back surface 3b of the member 3 and the surface 2a of the battery container 2 are intimately in contact with each other.
In this state, a welding current of a given value is supplied from a power source 8. The welding current is applied to the lead member 3 through one welding electrode. A part of the welding current is fed back to the power source 8 through the lead member 3 and the other welding electrode. The remaining part is applied to the region right under the distal end 7a of the one welding electrode and the surrounding area, and then flows across the thickness of the lead member 3 to reach the surface 2a of the battery container 2. Thereafter, the remaining current passes through the battery container 2, flows in the thickness direction of the lead member 3 around the region right under the distal end 7a of the other welding electrode, and is fed back to the power source through the other welding electrode.
In this process, Joule heat is generated in the contact interface between the back surface 3b of the lead member 3 and the surface 2a of the battery container 2 near the region right under each welding electrode, so that both members are partially melted to form nuggets near the contact interface. Thus, the lead member 3 is fixed to the battery container 2 by spot welding.
Macroscopically, the respective surfaces of the lead member and the battery container are flat surfaces. Microscopically, however, they are complicated irregular surfaces. When the lead member is put on the surface of the battery container, therefore, these two members are not uniformly in contact with each other. More specifically, infinitesimal projections on the back surface 3b of the lead member and the surface 2a of the battery container 2 are only in contact with each other""s surface. The formation of nuggets advances as current flows through the resulting infinitesimal contact portions to generate Joule heat therein.
As this is done, the welding behavior is complicated due to the influence of fine deformation of the lead member 3, which is caused as the welding electrodes 7 are pressed against the lead member 3, upon the contact state.
In the case where the battery container and the lead member are formed of a Ni-plated steel sheet, the following problems are believed to arise if they are welded by the parallel resistance welding method.
Increasing the strength of weld between the battery container and the lead member is pretty difficult and the range of operating conditions therefor is narrow. Moreover, the weld strength varies depending on the welding operation. This tendency is particularly remarkable when the lead member is welded to the bottom portion of a battery container that has a wide area for welding.
If the weld strength is not very high, then welded joints on a battery pack that is incorporated in an electric or electronic device to be actually used will possibly be broken by impact to ruin the function of the device in case the device is dropped, for example.
If the weld strength varies, moreover, it can be supposed to lower the reliability of welding of welded structures of manufactured batteries and lead members, in consideration of the fact that the welding operation is usually continuously performed in a production line.
In order to increase the strength of weld between the battery container and the lead member and reduce the variation of the strength to stabilize spot-welded joints, in the resistance welding method described above, it is necessary only that the Joule heat generated right under the two welding electrodes be intensified securely to form stable nuggets, basically.
To attain this, the generated Joule heat is intensified by setting the supplied current at a high value or lengthening the conduction time. If this is done, however, the welding electrodes sometimes may be fused to the surface of the lead member, thereby preventing composition of smooth welding processes. In some cases, furthermore, generation of excessive heat may results in production of melt dust of the metal material that constitutes the surface of the battery container and the lead member, thus lowering the weld strength, on the contrary.
An object of the present invention is to provide a battery and a lead member, designed so that high strength of weld between a battery container and the lead member can be maintained with less variation despite lower current supply thereto when the lead member is welded to the outer surface of the battery container by the parallel resistance welding method.
Another object of the invention is to provide a battery and a lead member, which require no high current supply for welding, so that welding electrodes cannot be easily fused to the lead member, and smooth welding processes can be composed without entailing production of melt dust.
Still another object of the invention is to provide a high-reliability battery pack that cannot be easily broken by external force such as impact.
In order to achieve the above objects, according to the present invention, there is provided a battery comprising a battery container and a layer of an Ni-based alloy covering a part or the whole of the outer surface of the battery container.
According to the invention, moreover, there is provided a lead member for battery connection, which is welded to the outer surface of a battery by the parallel resistance welding method, the lead member comprising a substrate and an Nixe2x80x94Sn alloy layer formed at least on that surface of the substrate which is welded to the outer surface of the battery.
According to the invention, furthermore, there is provided a battery pack comprising a group of the aforesaid batteries according to the invention connected to one another by means of lead members for battery connection by the parallel resistance welding method and a shell storing the group of the batteries therein, or a battery pack comprising a group of batteries connected to one another by means of the aforesaid lead members for battery connection according to the invention by the parallel resistance welding method and a shell storing the group of the batteries therein, or a battery pack comprising a group of the aforesaid batteries according to the invention connected to one another by means of the aforesaid lead members for battery connection according to the invention by the parallel resistance welding method and a shell storing the group of the batteries therein.